Numerical line current differential protection has been established as one of the most popular transmission line protection scheme. This is mainly because of its simple and clear nature. The differential line protection has natural phase segregated operation capability; immunes to power swing; weak in-feed; and is applicable to multi-terminals solution, etc. The developments of communication technology have even promoted more popular applications of line differential protection.
In a numerical line differential protection, the signals of sampled current are obtained from protection devices (e.g., IEDs) located geographically apart from each other. The current signals sampled from different IEDs (normally locate at different line ends) have to be synchronized (also referred to as time coordinated) before comparing with each other in order to avoid introducing errors.
The synchronization of sampled signals from different IEDs (also referred to as the synchronization of different IEDs in this invention) has to be very accurate, or otherwise the synchronization error might cause serious incorrect operation of line current differential protection. An inaccuracy of 0.1 ms in a 50 Hz AC system brings a maximum amplitude error of operation current around 3%, whilst an inaccuracy of 1 ms brings a maximum amplitude error of around 31%. The corresponding errors for a 60 Hz system are respectively around 4% and 38%. (See, ABB Technical reference manual line difference protection IED RED670, and Phil Beaumont, Gareth Baber, et al. Line Current Differential Relays Operating over SDH/SONET Networks. PAC, summer 2008).
Presently, most of line differential relays adopt the so called “Echo method” (also referred to as “Ping-Pong method”) to ensure the synchronization. The theory of echo method is briefly introduced by one echo process below:
As shown in FIGS. 1, A and B indicates two protection devices, such as IED. And the IEDs communicate with each other by sending and receiving messages. The IED B sends a message to IED A at IED B's internal time T1. IED A receives the message at its internal clock time T2. Similarly, IED A sends a message to IED B at IED A's internal time T3, and IED B receives the message at its internal time T4. Therefore, time instances T2 and T3 are taken with reference to the internal clock of IED A, and the time instances T1 and T4 are taken with reference to the internal clock of IED B. The communication time consumed between the IEDs is thereby measured.
The time instances T2 and T3 are transferred from IED A to IED B (or vice versa). It is assumed that the sending and receiving delays between the IEDs are equal to each other (also referred to as symmetrical channel delay). IED B then calculates the communication delay time Td (from IED A to IED B or from IED B to IED A) and the clock disparity Δt between the reference clocks of IED A and IED B.
The clock disparity Δt and communication delay are then used for synchronizing the sampled signals through interpolating the sampled signals from the remote end before the current differential algorithm is executed, or executing sampling timing control to achieve that the sampling instants of both IEDs are synchronized. The synchronization can be implemented by many conventional approaches, such as the method disclosed in Houlei Gao, Shifang Jiang, et al. Sampling Synchronization methods in digital current differential protection. Automation of electric power systems, Vol 20, September 1996. The calculated clock disparity Δt and communication delay must be very accurate to ensure the accuracy of synchronization.
However, the assumption of symmetrical channel delay in the traditional echo method above is not always valid. This is particularly true with the popular applications of SDH/SONET (Synchronous Digital Hierarchy/Synchronous Optical Network). The SDH/SONET is able to survive failures of the network by reconfiguring and maintaining services through the use of self-healing ring architectures. The self-healing or self-switching structures in a communication ring can be either “unidirectional” or “bi-directional”. In a unidirectional switching, only the faulted path is switched to the opposite direction whilst the non-faulted path keeps its original route. In a bi-directional switching, when there is a fault occurred in the ring, both the sending and receiving routes are switched to follow the same opposite direction along the ring. The difference is that the bi-directional switching will maintain an equal signal communication delays for the same switched sending and receiving paths, whilst unidirectional switching may introduce permanently asymmetrical communication delays through different sending and receiving routes. However, it has to be noted that the bi-directional switching does not introduce permanently asymmetrical communication delays, but the introduced transient asymmetrical delays could be 50 ms or even longer.
When the channel delays are asymmetrical (i.e. sending and receiving delays are different) and such an asymmetry is not detected by the IEDs, the traditional echo method, which is based on the assumption of symmetrical communication delay, is no longer valid, and the differential IEDs are under high risk of incorrect operation. Therefore, a reliable method to ensure the synchronization no matter the channel delays are symmetrical or not is of high importance and desirable.
Several approaches have been proposed to solve the problem of asymmetrical communication delay. A GPS (or other external clocks such as Compass/BeiDou Navigation Satellite System, Galileo, etc) based method was proposed. As an example of such external clock based methods, a GPS receiver module is embedded in each IED for synchronizing its local clock with the external clock. However, in practice, the GPS signals are not always perfectly or precisely received by the IEDs. The GPS antenna has to be installed with care. Otherwise the reception of GPS signal may be interrupted. Mal-operation (e.g. field engineers unintentionally disconnect the cable or antenna) or unfavorable environment (e.g. antenna close to coast is corroded by water, military GPS jamming) also result an unreliable signal receiving. In the situations described above, the IED will probably lose the capability of tolerating communication delay asymmetry.
As shown in above paragraphs, the existing methods are not reliable under many situations. Therefore, it is highly desirable to propose a method which can reliably detect channel delay asymmetry and ensure accurate synchronization no matter the channel delays are symmetrical or asymmetrical.